1. Field of the Invention
The present invention relates to an illuminating device for illuminating display elements used in a portable information apparatus, a cellular phone, and the like, a display device having the illuminating device, and an optical film used in the illuminating device.
2. Description of the Related Art
In recent years, a liquid crystal display device, which obtains a high definition color image with small power consumption, is employed as a display device used in a cellular phone, a mobile computer, and the like. The liquid crystal display element requires an illuminating device because the liquid crystal display element is a non-selfluminescent display element that does not emit light. A superluminescent white LED is often used as a light source in the illuminating device.
In particular, in a cellular phone, a reflection-type liquid crystal display device that has a large and bright opening or a double-side visible type liquid crystal display device that is capable of displaying image information on both front and backsides thereof is used. In those display devices, a front light or a back light using a white LED as a light source has been used as an illuminating device. The white LED has a configuration in which a blue LED emitting blue light is covered with a resin in which a yellow phosphor is dispersed, and green light or yellow light emitted from the phosphor is mixed with original blue light to obtain white light by a complementary color relationship (see, for example, JP 10-107325 A). The liquid crystal display device selects a necessary color from light, which is emitted from the white LED, using color filters of red (R), green (G), and blue (B) provided in a liquid crystal panel and a switching function of the liquid crystal elements and displays the selected color.
Then, the following configuration is known, in which a phosphor that is excited with light in a region not passing through a liquid crystal panel and emits light having a wavelength passing through the liquid crystal panel is provided between the output plane of a light guide body and the display panel, whereby the use efficiency of light from alight source is enhanced (see, for example, JP 2006-338901 A).
Further, as a light source for general illumination, a cold cathode tube, an incandescent electric lamp, and the like are known. For example, when a current flows through a cold cathode tube, electrons jump from a fluorescent tube filament, and bump into mercury in gaseous form sealed inside, whereby UV-rays are emitted. The inside of the fluorescent glass tube is coated with a phosphor, and the phosphor emits light when UV-rays strike the phosphor, and visible light is released outside of the fluorescent tube to be used for illumination. That is, spectrum of light to be output is determined depending upon the characteristics and compounding of phosphors applied to the inside of the glass tube.
A wavelength distribution of the conventional white LED used in the illuminating device of the liquid crystal display device spreads broadly with peaks at 450 nm and 580 nm because light emitted by the white LED is white light of a mixed color obtained by mixing blue color light with yellow or green color light. On the other hand, peaks of wavelength selected by a color filter used in the liquid crystal display device or the like are 450 nm for blue, 530 nm for green, and 600 nm for red. In other words, among light from a white light source, the light having a wavelength of 480 nm to 510 nm and a wavelength 570 nm to 590 nm are absorbed by the color filter. Therefore, the use efficiency of light is low, and brightness is low. Further, according to the configuration described in JP 2006-338901 A, all light having the wavelengths to be absorbed by the liquid crystal panel does not necessarily strike the phosphor to be color-converted, with the result that the use efficiency cannot be enhanced to a desired degree. It is necessary to efficiently convert light having unnecessary wavelengths into light having a necessary wavelength.
On the other hand, for example, in the case where there is an object to be illuminated, a red component of which is desired to be emphasized, such as an apple, a green-based component is not necessary in illumination in a store and the like. A conventional light source for illumination does not have a function of eliminating only light having unnecessary wavelengths and emphasizing light having a necessary wavelength more. Therefore, it is not easy to eliminate only a green-based component and emphasize a red color more. For example, it may not be impossible to eliminate only a green-based component and emphasize a red color more, by using a color filter containing a pigment or the like cutting the periphery of a green-based component in a very high concentration, or using a plurality of light sources. However, there are problems of power consumption, heat generation, and a cost, and hence, the above method is unrealistic.